1. Field of the Invention
The present invention relates in general to an oil pump incorporated with an internal combustion engine, and more particularly to an improvement in a driving mechanism through which the oil pump is driven by a crankshaft of the engine.
2. Description of the Prior Art
In order to clarify the task of the present invention, one conventional oil pump disclosed in Japanese Utility Model First Provisional Publication 60-155709 will be described, which is shown in FIGS. 4 to 6 of the accompanying drawings. The oil pump disclosed is of a type which is directly driven by a crankshaft of an internal combustion engine.
FIG. 4 shows a lubrication system of a four cylinder type internal combustion engine 30 to which the oil pump 31 is connected. The oil pump 31 is directly driven by a crankshaft 35 of the engine 30. During operation of the oil pump 31, lublication oil flows in a manner as indicated by the arrows shown in the drawing. That is, during operation, oil in an oil pan (not shown) is pumped up through an oil strainer 32 where large particles are trapped, and the oil from the pump 31 flows to an oil filter 33 where fine particles are trapped, and the oil thus cleaned flows into a main gallery 34 from which the oil is supplied to main bearings of the crankshaft 35 and to crank journals 36a to 36e of the same. The oil which has lublicated these parts returns to the oil pan. A part of the oil which has been fed to the crank journals 36a to 36e flows through a passage of the crankshaft 35 to crank pins 37a to 37d to lublicate the same. A part of the oil which has been fed to the crank pins 37a to 37d is injected from splay openings of connection rods to lubricate pistons (not shown) in corresponding cylinders. While, a part of the oil which has been fed to the main gallery 34 is fed through a passage 38 to bearings of a cam shaft 39 and then to a rocker shaft 40 to carry out lubrication of a valve mechanism 41 and a distributor 42. The oil from these parts is introduced through oil inlets 43 to 43c into interior passages of a cylinder block and then returns to the oil pan. A part of the oil which has been fed to the main gallery 34 is injected from splay openings of a chain tensioner 44 to lubicate a chain 45, and injected from a pipe 46 to lubricate a chain sprocket wheel 47 and the chain 45. The oil from these parts returns to the oil pan.
As is seen from FIG. 5, the oil pump 31 disclosed by the above-mentioned publication is of a rotor type which comprises an annular body 50, an internal gear 51 rotatably held in the body 50 and an external gear 52 eccentrically received in and partially meshed with the internal gear 51 leaving a crescent member 53 therebetween. The body 50 is formed at the diametrically opposed portions thereof with inlet and outlet ports 54 and 55. The external gear 52 is formed at its central portion with a semicircular bore 52a into which a semicircular portion 58 (see FIG. 6) of the crankshaft 35 is snugly and latchingly received. Under operation of the engine 30, the external gear 52 is driven by the crankshaft 35 and the internal gear 51 is driven by the external gear 52, so that oil is forced to flow in a known manner from the inlet port 54 to the outlet port 55 under pressure.
The motion transmittance from the crankshaft 35 to the oil pump 31 is carried out through the parts which are illustrated in FIG. 6. That is, the crankshaft 35 is formed at its leading end with a crank journal portion 56, a sprocket wheel mounting portion 57, a semicircular portion 58 and a pulley mounting portion 59 which are coaxially arranged in this order and the diameters of which are stepwisely reduced in this order, as shown. A chain sprocket wheel 47 is coaxially mounted on the mounting portion 57 and splined to the same. For this splined-connection, the sprocket wheel 47 and the mounting portion 57 have respective key grooves 47a and 57b. The inner diameter D.sub.1 of the sprocket wheel 47, which is substantially equal to the diameter D.sub.3 of the sprocket wheel mounting portion 57, is larger than the diameter D.sub.2 of the semicircular portion 58 so as to permit insertion of the sprocket wheel 47 onto the mounting portion 57 from the leading end of the crankshaft 35. The sprocket wheel 47 meshes with a drive chain 45 (see FIG. 4) so as to transmit rotation of the crankshaft 35 to a cam shaft 39 at a one half speed reduction. For conformity with the semicircular bore 52a of the external gear 52, the semicircular portion 58 of the crankshaft 35 has two diametrically opposed flat portions, as shown.
However, the above-mentioned motion transmitting parts have the following drawbacks due to their inherency in construction.
That is, the chain sprocket wheel 47 is bulky in construction because the same must have an inner diameter D.sub.1 greater than the diameter D.sub.2 of the semicircular portion 58 which has a considerable size for assuring the motion transmittance therefrom to the external gear 52 of the oil pump 31. Bulky construction of the sprocket wheel 47 causes increase in the number of teeth 47b formed thereabout, and thus induces increase in the number of teeth formed about a cam shaft sprocket wheel 39a (see FIG. 4) by which the rotation speed of the cam shaft 39 is reduced to one half as compared with that of the crankshaft 35. Employment of these two bulky sprocket wheels 47 and 39a causes increase in size of a chain cover on which the oil pump 31 is integrally mounted, and thus the height of the engine is inevitably increased, which counters the general requirement for engine compactness.